Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for transmitting acknowledgement (ACK) in a Wireless Local Area Network (WLAN) using multiple antennas.
Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a Wireless Local Area Network (WLAN) is a technique for wirelessly accessing the Internet at homes or companies or in specific service providing areas by using portable terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), based on wireless frequency technology.
A lot of standardization tasks are being performed since Institute of Electrical and Electronics Engineering (IEEE) 802 (i.e., the standardization organization of WLAN technology) was established on February, 1980.
WLAN technology initially supported a speed of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a frequency of 2.4 GHz according to IEEE802.11, but recently may support a maximum speed of 54 Mbps by using Orthogonal Frequency Division Multiplexing (OFDM). In addition, in IEEE802.11, standardizations for various techniques, such as the improvement of Quality of Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, are being put to practical use or developed.
IEEE802.11b of IEEE802.11 uses a frequency of 2.4 GHz band and also supports a maximum communication speed of 11 Mbs. IEEE802.11a commercialized since IEEE802.11b has reduced the influence of interference as compared with the frequency of a 2.4 GHz band by using a frequency of a 5 GHz band not the 2.4 GHz band that is very complex and has improved the communication speed to a maximum of 54 Mbps by using OFDM technology. However, IEEE 802.11a is disadvantageous in that it has a shorter communication distance than IEEE 802.11b. Furthermore, IEEE 802.11g embodies a maximum communication speed of 54 Mbps by using the frequency of 2.4 GHz band like IEEE 802.11b. IEEE 802.11g has been greatly in the spotlight because it satisfies backward compatibility, and it also has a better communication distance than IEEE 802.11a.
Furthermore, in order to overcome a limit to the communication speed that was considered as being weakness in a WLAN, IEEE 802.11n has recently been established as a technical standard. An object of IEEE 802.11n is to increase the speed and reliability of a network and to extend the coverage of a wireless network. More particularly, in order to support a High Throughput (HT) having a maximum data processing speed of 540 Mbps or higher, minimize an error in transmission, and optimize the data speed, IEEE 802.11n is based on Multiple Inputs and Multiple Outputs (MIMO) technology using multiple antennas on both sides of a transmitter and a receiver. Furthermore, this standard may use not only a coding scheme for transmitting several redundant copies in order to increase data reliability, but also Orthogonal Frequency Division Multiplex (OFDM) in order to increase the speed.
As the propagation of a WLAN is activated and applications using the WLAN are diversified, there is a need for a new WLAN system for supporting a throughput higher than the data processing speed supported by IEEE 802.11n. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which are recently proposed in order to support a data processing speed of 1 Gbps or higher. The term ‘VHT WLAN system’ is arbitrary, and a feasibility test on a system using 8×8 MIMO and a channel bandwidth of 80 MHz or higher is in progress in order to provide the throughput of 1 Gbps or higher.
For more reliable data transmission, there is a scheme in which a receiver that has received a data frame from a transmitter transmits acknowledgement (ACK). The ACK is used as block ACK in a WLAN. For the block ACK, reference may be made to Paragraph 7.2.1 of “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, Amendment 5: Enhancements for Higher Throughput” of IEEE 802.11n/D11.0 disclosed on June, 2009. A transmitter transmits a plurality of data frames. A receiver transmits a block ACK frame to the transmitter as ACK to the plurality of data frames.
However, a VHT WLAN system adopts Multi-User (MU)-MIMO in which a plurality of stations (STAs) accesses a channel at the same time and exchanges frames with an AP in order to use the channel more efficiently. In a WLAN system supporting MU-MIMO, how the ACK frame, that is, ACK, will be transmitted may be problematic.
Conventional data frame transmission and an ACK frame in response to the data frame are based on condition that frames are exchanged between an AP and a single STA. If the conventional method is applied to a WLAN system supporting MU-MIMO without change, it may be inefficient.